Investigation of the basic mechanisms involved in RNA polymerase III (pol III)-mediated RNA expression in eukaryotes was continued. Pol III is responsible for the synthesis of tRNA and 5S rRNA. Viral infection and viral transforming proteins work in part through their effects on components of the pol III transcription apparatus. In addition to the obvious importance of the role of transcription in supplying the cell with essential small RNAs, molecular dissection of pol III transcription also provides a model for the investigation of the other eukaryotic RNA polymerases. Pol III-mediated expression of human Alu transposons was also investigated. Alu repetitive elements occupy nearly one million sites in the human genome, having been inserted by a poorly understood retroposition process that is responsible for de novo insertional mutagenesis in humans. Although viral infection and certain forms of cell stress lead to a dramatic induction of Alu RNA,the mechanisms that govern Alu RNA expression are largely unknown. Small Alu RNAs form discrete ribonucleoprotein particles by binding to the signal recognition particle (SRP) protein SRP9/14, a protein normally involved in translational control of nascent proteins that are destined for secretion through the endoplasmic reticulum. 1. Alu RNAs induced upon infection of human cells with adenovirus are associated with the SRP9/14 protein in the form of stable ribonucleoprotein (RNP) complexes. 2. SRP9/14 has played a major role in Alu transposition in the human genome. 3. A highly conserved nucleotide in the Alu domain of SRP RNA (and Alu RNA) mediates translation arrest through high efficiency binding to SRP9/14. 4. Several site-directed mutant forms of the human La antigen were examined in structural-functional assays and revealed a complex structure for the this pol III transcription factor. 5. The site on the human La antigen that is phosphorylated in vivo was mapped to serine 366, within an evolutionarily conserved casein kinase II (CKII) target site, and this was shown to regulate the activity of pol III transcription complexes. 6. A novel mechanism of eukaryotic transcriptional regulation was elucidated: Control at the level of recycling of transcription complexes. 7. Eukaryotic tRNA expression can be studied in the fission yeast Schizosaccharyomyces pombe.